A. FIELD OF THE INVENTION
This invention relates to synthetic dolomite and more particularly to a process for the production of synthetic double burnt dolomite.
B. DESCRIPTION OF RELATED ART
The dead burnt or sintered dolomite has a wide range of applications such as a low cost refractory material for the manufacture of refractory bricks used in electric and open hearth furnaces, as well as in furnaces for iron melting.
The dolomite is a mineral mainly including calcium carbonate (CaCO.sub.3) and magnesium carbonate (MgCO.sub.3) in a relation of approximately 60% by weight: 40% by weight respectively.
The grade or quality of the raw dolomite depends on the different proportions of silica (SiO.sub.2), iron (Fe.sub.2 O.sub.3), and alumina (Al.sub.2 O.sub.3), contained in the natural mineral, so that the sum of such impurities should be up to about 1% for a high grade sintered dolomite.
The desired refractory properties of the dolomite for the manufacturing of refractory bricks, depend on the impurity level of the dolomite.
The use of dolomite refractory has been limited by the tendency of the oxides, especially lime, to revert to the hydroxide form upon exposure to moisture in the atmosphere.
High porosity and low density sintered material in the dolomite refractory are highly affected by steel slag lowering its refractory advantages.
The use of certain additives such as oxides and fluxing agents have been proposed to improve the main characteristics of low porosity, high density and a low hydration susceptibility of the dolomite refractory.
To prepare the dolomite for the production of refractory bricks, the natural high grade dolomite is passed over a sintering process consisting in heating the dolomite to a temperature of 1,800.degree. C. to 2,000.degree. C.
With this process the dolomite density (Bulk Specific Gravity) is increased and the impurity level helps to stabilize the produced calcium oxide.
High grade natural dolomite having the desired refractory properties for the manufacture of brick, can only be obtained from certain locations throughout the world, such as in the United States.
This leads other brick producing countries to buy such high grade dolomite, which is not always available, and transport it to their facilities, increasing the price of the products.
There are many places in the world having low impurity content dolomite, i.e. having a deficiency of silica (SiO.sub.2), iron (Fe.sub.2 O.sub.3), and alumina (Al.sub.2 O.sub.3), from which, as mentioned, depend the desired properties of the dolomite.
Therefore, it would be highly desirable to process the low impurity content dolomite, to obtain the desired levels of impurities in a synthetic high degree sintered dolomite.
It has been found that the sum of such impurities in the sintered material should be of about 1% by weight to 2% by weight.
Such sintering process, however, has to be economically viable in order that it does not result in a more expensive product than the natural high grade dolomite.
Different processes have been developed to produce a low porosity and high stabilized dead burnt dolomite having an improved hydration resistance.
Representative processes for the production of synthetic dolomite are disclosed in the following U. S. Patents:
U.S. Pat. No. 4,394,454, discloses a method for making a dolomite sinter with low porosity and good hydration stability, by adding a member selected from burnt dolomite, dolomite hydrate, semi burnt dolomite and combinations thereof, to raw dolomite in order to introduce foreign oxides thereto, pressing the milled raw dolomite into briquettes and heating the briquettes to a sintered temperature.
U.S. Pat. Nos. 5,246,648 and 4,627,948 disclose a process including deacidification steps using a specific apparatus.
U.S. Pat. Nos. 4,648,966; 4,636,303 and 4,372,843 disclose processes for the benefit of phosphate minerals, including some flotation steps and involving carbonate collectors with sulfated fatty acids.
U.S. Pat. No. 5,122,350 discloses a process for the production of a specific acetate.
However, in all of these treatment processes to transform low grade dolomite into a high grade synthetic dolomite, by introducing the above-referenced impurities as additives, the dolomite obtained still did not show the desired properties for refractory brick production, i.e., high bulk specific gravity and low hydration susceptibility.
It has been found that the process for the production of synthetic dolomite depends on the way in which such impurities are introduced into low grade dolomite, including some burning and reacting steps, as well as the introduction of some very specific additives, to allow the integration of the above-referenced impurities.
It has been additionally found very specific additives, such as Fe.sub.2 O.sub.3, SiO.sub.2 and Mg(OH).sub.2, Ca(OH).sub.2 or combinations of both, that could be added to the natural dolomite if it does not contain the necessary impurity level in order to balance the impurities for its stabilization, and guarantee the desired properties.
The process of the present invention for the production of synthetic dolomite, comprises: grinding a dry and low impurity level mineral dolomite; incorporating additives selected from the group consisting of iron as Fe.sub.2 O.sub.3, silica (SiO.sub.2) and magnesium hydroxide Mg(OH).sub.2, calcium hydroxide Ca(OH).sub.2 or a combination of both, in order to react and integrate the dolomite impurities as the desired silica (SiO.sub.2), iron (Fe.sub.2 O.sub.3), and alumina (Al.sub.2 O.sub.3), in the dolomite composition; wholly mixing these additives with the ground dolomite in a mixer machine; compacting the mixed materials in order to form briquettes; sieving the briquettes of compacted material to eliminate fines; burning the briquettes to a temperature of 1,700.degree. C. to 2,200.degree. C. to guarantee a complete sintering; cooling the briquettes and sieving the cooled briquettes.